As the data areal density in hard disk drive (HDD) writing increases, write heads and media bits are both required to be made in smaller sizes. However, as the write head size shrinks, its writability degrades. To improve writability, new technology is being developed that assists writing to a media bit. Two main approaches currently being investigated are thermally assisted magnetic recording (TAMR) and microwave assisted magnetic recording (MAMR) where a spin torque device is employed in the write gap to generate a high frequency field that helps writing. The latter is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008). A third approach called STRAMR (spin torque reversal assisted magnetic recording) relies on spin torque to reverse a magnetization in a layer in the write gap (WG), for example, to increase reluctance and force more magnetic flux from the MP at the ABS. STRAMR is described in U.S. Pat. No. 6,785,092. Related patent application Ser. No. 16/209,151 describes a writer where the MAMR and STRAMR (spin flipping element) effects may exist simultaneously.
Spin transfer torque devices (also known as STO devices) are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the first ferromagnetic layer (FM1) will generate spin polarized currents as the electrons traverse FM1. When the spin polarized current is transmitted through a polarization preservation spacer, the spin angular moment of electrons incident on a second FM layer (FM2) interacts with magnetic moments of FM2 near the interface between the non-magnetic spacer and FM2. Through this interaction, the electrons transfer a portion of their angular momentum to FM2. As a result, spin-polarized current can switch the magnetization direction of FM2 if the current density is sufficiently high.
Spin Hall Effect (SHE) is a physics phenomenon discovered in the mid 20th century, and is described by M. Dyaknov et al. in Physics Lett. A, Vol. 35, 459 (1971). Similar to a regular Hall Effect where conduction carriers with opposite charges are scattered to opposite directions perpendicular to the current density due to a certain scattering mechanism, SHE causes electrons with opposite spins to be scattered to opposite directions perpendicular to the charge current density as a result of strong spin-orbit coupling in the conducting layer. As shown in FIG. 1, electrons pass through a non-magnetic conductor 2 with strong spin orbit interaction, and electrons 3a with spin in the negative x-axis direction are deflected to the +z-axis surface 2s1 while electrons 3b with spin in the positive x-axis direction are deflected to the negative z-axis surface 2s2. SHE is quantified by the Spin Hall Angle (SHA) defined as the ratio of the spin current in the direction transverse to the charge current (z-axis in FIG. 1) to the charge current (y-axis direction in FIG. 1). For many years after SHE was discovered, the absolute value of SHA materials evaluated was typically less than 0.01, and SHE had very limited application in industry.
During the past 10 years, materials with substantially larger (giant) SHA have been found. B. Gu et al. in Phys. Rev. Lett. 105, 216401 (2010), and L. Liu et al. in Phys. Rev. Lett. 106, 036601 (2011) provided examples of SHA˜0.07 in a Pt layer, and as large as 0.12 in a Au layer with Pt doping, and an application where giant transverse spin current is injected into an adjacent magnetic layer to induce reversal and ferromagnetic resonance by spin torque. A large but negative SHA of around −0.12 was found in β-Ta, meaning that electrons in the β-Ta layer are spin scattered in the opposite directions compared to what is shown in FIG. 1. SHE with the negative SHA material was also used to interact with an adjacent magnetic layer, and even flip a magnetization in a magnetic layer in a magnetic random access memory (MRAM) device without sending a current through the magnetic tunnel junction. The aforementioned applications using SHE, or spin orbit torque (SOT) in MRAM, are typically called SOT-MRAM, and can significantly reduce the reliability concern that is generally found in conventional spin torque transfer (STT)-MRAM.
All existing designs that assist magnetic recording have advantages, but also have disadvantages including a greater number of pads per head for all assisted designs, reliability concern for TAMR, and a limited WG thickness to fit in a multilayer device in both MAMR and TAMR. In a first SHAMR design that we disclosed in related application Ser. No. 16/370,634, we found that applied current (ISHE) that is confined within a SHE layer can cause heating that leads to SHE layer protrusion up to 1-2 nm out of the ABS. Furthermore, synchronization of ISHE with Iw is difficult because of the required frequency in the GHz regime. To address the heating and synchronization issues, a second SHAMR design with SHE1 and SHE2 layers was disclosed in Ser. No. 16/563,147 where a first current (I1) flows between the MP and SHE1, and a second current (I2) flows between SHE2 and the TS. However, we subsequently found that while local charge current distribution at the MP/SHE1 and SHE2/TS interfaces provides a desirable assist proximate to the ABS, there may be a slightly negative assist from a back portion of the SHE layers that is greater than 20 nm from the ABS, for example. Therefore, a new SHE assist scheme is desired that solves all of the aforementioned concerns with earlier SHAMR designs while maintaining the benefits of the earlier schemes.